Catalysts for polymerization of polyolefin in the related art may be classified into Ziegler-Natta type catalysts, chromium-based catalysts and metallocene catalysts according to the kind of central metal. These catalysts are different from each other in catalyst activity, molecular weight distribution characteristics of a polymer and reaction characteristics for comonomers and thus are selectively used according to each preparation process and applied product.
Among them, Ziegler-Natta type catalysts are most commonly used and may be classified into magnesium-supported catalysts and silica-supported catalysts according to kinds of supports thereof.
There are two methods for preparing the magnesium-supported catalyst, and the first is a method, including: reacting a magnesium compound with an electron donor such as alcohol, amine, ether, ester, carboxylic acid and the like to prepare a magnesium compound solution, preparing the magnesium compound solution thus prepared into a solid form magnesium compound by a method such as spray drying, vacuum drying, and the like, and then reacting the compound with a titanium compound to finally prepare a solid catalyst for polymerization. And, the second is a method, including: reacting the magnesium compound solution in a solution state with a titanium compound to prepare a solid catalyst.
However, since a spherical solid catalyst support is first prepared by a method such as spray drying and the like in the first method, it is advantageous in that the final catalyst may also be produced in a spherical solid shape, but disadvantageous in that the first method includes more complicated preparation processes than the second method and has a wide particle size distribution of a manufactured solid catalyst.
Accordingly, many studies have been recently performed on the second method of obtaining a solid catalyst by directly reacting a magnesium compound solution with a titanium compound, which has a simple preparation process compared to the spray drying method.
In this regard, U.S. Pat. No. 6,323,150 describes a method for preparing a catalyst, by subjecting a magnesium compound to a primary treatment with a titanium compound including a polycarboxylic acid ester compound to obtain a solid-phase magnesium compound, heating the magnesium compound at from 70° C. to 130° C. to separate solid components and re-treating the separated solid components with the titanium compound. However, this technology is disadvantageous in that a substantially great amount of a titanium halide compound needs to be used during the preparation process, the process thereof is also complicated, and polypropylene (a propylene polymer) polymerized by using the same has a low bulk density of 0.45 g/ml or less.
U.S. Pat. No. 2007/0298964 relates to a method for preparing a solid titanium catalyst for alpha-olefin polymerization, and the method includes: dissolving a magnesium halide compound in a mixed solvent of cyclic ether and one or more alcohols, reacting the resulting magnesium compound solution with hydrocarbon including a titanium alkoxy halide compound and a halogen element to prepare a support, and reacting the prepared support with a titanium compound and an electron donor to prepare a solid complex titanium catalyst for polymerization of alpha-olefin. However, it is disadvantageous in that the polypropylene polymerized by using a catalyst prepared by this technology also has a bulk density of 0.43 g/ml or less.